I installed Ikonboard on my site and assisted in the set up on a non-profit site I manage. Ikon is free, but I highly recommend paying the $39 for them to install it vs doing it yourself. Less grief for one.
UBB by InfoPop is good, if you can afford it. I installed their demo. But I got more answers from the massive Ikonboard board than I did at Infopop.
Ikonboard prides themselves on assisting people hack into their board.
As for accessibility, Ikonboard is better. UBB is getting there but slowly.
Where are we going and why are we in this basket?
mine - http://holyroller.org
dog's - http://joella.holyroller.org